The human body is composed of tissues that are generally opaque. In the past, exploratory surgery was one common way to look inside the body. Today, physicians can use a vast array of imaging methods to obtain information about a patient. Some non-invasive imaging techniques include modalities such as X-ray, magnetic resonance imaging (MRI), computer-aided tomography (CAT), ultrasound, and so on. Each of these techniques has advantages that make it useful for observing certain medical conditions and parts of the body. The use of a specific test, or a combination of tests, depends upon the patient's symptoms and the disease being diagnosed.
MRI was established as a medical diagnostic technique that offers high-resolution anatomical information about the human body, and has since been used for the detection of a multitude of diseases. MRI creates images of a body using the principles of nuclear magnetic resonance. MRI can generate thin-section images of any part of the body from any angle and/or direction, in a relatively short period of time, and without surgical invasion. MRI can also create “maps” of biochemical compounds within any cross section of the body.
MRI is possible in the human body because the body is filled with small biological magnets—the most important, for MRI purposes, being the nucleus of the hydrogen atom, also know as a proton. Once a patient is placed into a MRI unit, their body is placed in a steady magnetic field that is more than 30,000 times stronger than the Earth's magnetic field. The MRI stimulates the body with radio waves to change the steady-state orientation of the protons, causing them to align with the magnetic field in one direction or the other. The MRI then stops the radio waves and “listens” to the body's electromagnetic transmissions at a selected frequency. The transmitted signal is used to construct images of the internal body using principles similar to those developed for computerized axial tomography scanners (CAT scanners). Since the nuclear magnetic relaxation times of tissues and tumors differ, abnormalities can be visualized on the MRI-constructed image.
The continued use and development of MRI has stimulated interest in the development of contrast agents capable of altering MRI images in diagnostically useful ways. Contrast agents that are currently favored by researchers in the field are suitably complexed paramagnetic metal cations. The use of contrast agents in MRI imaging offers major opportunities for improving the value of the diagnostic information which can be obtained.
Radio contrast agents, which are used in radioisotopic imaging in a manner analogous to MRI contrast agents, are a well-developed field. The knowledge existing in this field thus provides a starting point for the development of MRI contrast agents. MRI contrast must meet certain characteristics, however, which are either not required or are considerably less critical in the case of radio contrast agents. MRI contrast agents must be used in greater quantities than radio contrast agents. As a result, they must not only produce detectable changes in proton relaxation rates but they must also be (a) substantially less toxic, thereby permitting the use of greater amounts; (b) more water soluble to permit the administration of a higher dosage in physiologically acceptable volumes of solution; and (c) more stable in vivo than their radiopharmaceutical counterparts. In vivo stability is important in preventing the release of free paramagnetic metals and free ligand in the body of the patient, and is likewise more critical due to the higher quantities used. For the same reasons, MRI contrast agents that exhibit whole body clearance within relatively short time periods are particularly desirable.
Since radio contrast agents are administered in very small dosages, there has been little need to minimize the toxicity of these agents while maximizing water solubility, in vivo stability and whole body clearance. It is not surprising therefore that few of the ligands developed for use as components in radio contrast preparations are suitable for use in preparation of MRI contrast agents. A notable exception is the well known ligand diethylene triamine pentaacetic acid (DTPA), which has proved useful in forming complexes with both radiocations, pharmacologically suitable salts of which provide useful radio contrast agents, and paramagnetic cations such as gadolinium, whose pharmacologically suitable salts have proved useful as MRI contrast agents.
The contrast agents used in MRI derive their signal-enhancing effect from the inclusion of a material exhibiting paramagnetic, ferromagnetic, or superparamagnetic behavior. These materials affect the characteristic relaxation timers of the imaging nuclei, primarily water, in the body regions into which they distribute causing an increase or decrease in magnetic resonance signal intensity. There is therefore a long felt need for an MRI imaging agent which is substantially non-toxic, highly water soluble, and highly stable in vivo and which is capable of selectively enhancing signal intensity in particular tissue types.
Optical imaging continues to gain more acceptance as a diagnostic modality since it is similar to MRI and does not expose patients to ionizing radiation. Optical imaging is based on the detection of differences in the absorption, scattering and/or fluorescence of normal and tumor tissues. One type of optical imaging comprises near-infrared fluorescent (“NIRF”) imaging. Generally, in NIRF imaging, filtered light or a laser with a defined bandwidth is used as a source of excitation light. The excitation light travels through the body and when it encounters a NIRF molecule or optical imaging agent, the excitation light is absorbed. The fluorescent molecule (i.e., the optical imaging agent) then emits detectable light that is spectrally distinguishable from the excitation light (i.e., they are lights of different wavelengths). Generally, light that is detectable via NIRF imaging has a wavelength of approximately 600-1200 nm. This is important because at these wavelengths tissue autofluoresence and scattering is minimal, allowing for deep tissue imaging not capable at other wavelengths. The optical imaging agent increases the target:background ratio by several orders of magnitude, thereby enabling better visibility and distinguishability of the target area. Optical imaging agents can be designed so that they only emit detectable light upon the presence of a particular event (i.e., in the presence of a predetermined enzyme). Optical imaging, such as NIRF imaging, shows significant promise for detecting functional or metabolic changes in deep tissues, such as the overproduction of certain proteins or enzymes, in a body. This is useful because the majority of diseases induce early functional or metabolic changes in the body before anatomical changes occur. The ability to detect these metabolic changes allows for early detection, diagnosis and treatment of a disease, thereby improving the patient's chance of recovery and/or of being cured.
A contrast agent is often used in conjunction with MRI and/or optical imaging to improve and/or enhance the images obtained of a person's body. A contrast agent is a substance that is introduced into the body to change the contrast between two tissues. Generally, MRI contrast agents comprise magnetic probes that are designed to enhance a given image by affecting the proton relaxation rate of the water molecules in proximity to the MRI contrast agent. This selective change of the T1 (Spin-Lattice Relaxation Time) and T2 (Spin-Spin Relaxation Time) of the tissues in the vicinity of the MRI contrast agents changes the contrast of the tissues visible via MRI. Generally, optical contrast agents comprise dyes designed to emit light when excited with the correct wavelength light. This emitted light is then detected by an optical imaging device.
Contrast agents are administered to a person, typically via intravenous injection into their circulatory system, so that abnormalities in the person's vasculature, extracellular space and/or intracellular space can be visualized. Some contrast agents may stay in the person's vasculature and highlight the vasculature. Other contrast agents may penetrate the vessel walls and highlight abnormalities in the extracellular space or intracellular space through different mechanisms, like, for example, binding to receptors. After a contrast agent is injected into a tissue, the concentration of the contrast agent first increases, and then starts to decrease as the contrast agent is eliminated from the tissue. In general, a contrast enhancement is obtained in this manner because one tissue has a higher affinity or vascularity than another tissue. For example, most tumors have a greater MRI contrast agent uptake than the surrounding tissues, due to the increased vascularity and/or vessel wall permeability of the tumor, causing a shorter T1 and a larger signal change via MRI.
Typical MRI contrast agents belong to one of two classes: (1) complexes of a paramagnetic metal ion, such as gadolinium (Gd); or (2) coated iron nanoparticles. As free metal ions are toxic to the body, they are typically complexed with other molecules or ions to prevent them from complexing with molecules in the body, thereby lessening their toxicity. Some typical MRI contrast agents include, but are not limited to: Gd-EDTA, Gd-DTPA, Gd-DOTA, Gd-BOPTA, Gd-DOPTA, Gd-DTPA-BMA (gadodiamide), feruimoxsil, ferumoxide and ferumoxtran.
Another class of MRI contrast agents—called “smart” contrast agents—includes contrast agents that are activated by the physiology of the body or a property of a tumor, i.e., agents that are activated by pH, temperature and/or the presence of certain enzymes or ions. Some examples of MRI smart contrast agents include, but are not limited to, contrast agents that are sensitive to the calcium concentration in a body, or those that are sensitive to pH.
“Smart” optical contrast agents have recently been used in vivo to monitor enzyme activity in tissues. These smart contrast agents only produce contrast in the presence of specific proteases. Since proteases are key factors involved in multiple disease processes, the ability to tailor contrast agents or probes to specific enzymes should ultimately allow one to detect the expression levels of marker enzymes for various pathologic conditions. This approach is capable of providing all the necessary information for studying pathologies near the surface of the skin via optical imaging. However, since low localization information is characteristic of optical imaging, one or more additional modalities may be required for diagnosing pathologies deeper within the body.
Contrast agents are often required in order to make the presence of certain diseases detectable. For example, the mechanisms of contrast in MRI (such as T1, T2 and/or proton density) are somewhat limited, allowing certain diseases to remain undetectable by MRI in the absence of exogenous contrast agents. This is because none of the parameters that influence contrast are affected in some diseases without the addition of a contrast agent. Therefore, using contrast agents in conjunction with MRI offers excellent sensitivity for detecting some additional pathologic conditions, thereby allowing some diseases to be detected that would otherwise be undetectable via MRI alone. For example, MRI in the presence of contrast agents has very high sensitivity for detecting breast tumors, but very low specificity for the detection of cancerous tissue. The specificity for identifying cancerous tissue is so low via MRI because multiple pathologies, such as the recruitment and production of new blood vessels (angiogenesis), are characterized by markers similar to those of cancerous tissue.
While both MRI and optical imaging provide useful information, neither independently provides all the information desired to help make early diagnoses of all diseases. As previously discussed, the majority of diseases induce early functional or metabolic changes in the body before anatomical changes occur. While these metabolic changes are almost impossible to detect via current MRI techniques, optical imaging shows significant promise in being able to detect such changes. However, when applications such as breast imaging are envisioned, optical imaging by itself is very limited by the spatial resolution that can be achieved.
Nanoparticle probes have found tremendous success in recent years as labels in biological systems and have shown great potential for bioimaging (Akerman et al., Proc Natl Acad Sci USA 2002, 99:12617; Santra et al., Analytical Chemistry, 2001, 73:4988; Santra et al., Journal of Biomedical Optics, 2001, 6:160; Ben-Ari et al., Journal of the National Cancer Institute, 2003, 95:502; Panyam et al., International Journal of Pharmaceutics, 2003, 262:1); diagnostic (Brigger et al., Adv Drug Deliv Rev, 2002, 54:631; Alivisatos, Scientific American, 2001, 285:66), and therapeutic purposes (Emerich et al., Expert Opinion on Biological Therapy 2003, 3:655; Douglas et al., Crc Critical Reviews in Therapeutic Drug Carrier Systems, 1987, 3:233; Holm et al., Molecular Crystals and Liquid Crystals, 2002,374:589).
Recently, the demand for the development of multimodal nanoparticle probes for in vivo disease diagnosis and therapy has increased significantly. Multimodal nanoparticle probes have the potential for imaging cells, tissues and other organs in multiple modes, as well as the delivery of therapeutic agents to specific targets.
Diagnostic neuroimaging techniques such as angiography, CT (computed tomography) and MRI are widely used to monitor changes in anatomy and disease diagnosis (Hildebrandt et al., Clinical Immunology, 2004, 111:210; Dzik-Jurasz et al., British Journal of Radiology, 2004:77, 296; Costouros et al., Journal of Cellular Biochemistry, 2002:72; Langer et al., World Journal of Surgery, 2001, 25:1428; Smith et al., Journal of Neurotrauma, 1995, 12:573; Kreel et al., Postgraduate Medical Journal, 1991, 67:334). Contrast agents are often administered to patients, to help delineate pathological from healthy tissue. Contrast agents for angiography and CT scans are radio-opaque, which allow clear visualization of the contrast under an X-ray source. Iodinated chemical compounds such as iohexol (also called Omnipaque™) and iodixanol (also called Visipaque™) are routinely used as X-ray contrast agents. They consist of electron dense iodine atoms, which show contrast under an X-ray. MRI contrast agents such as Gadoteridol, (Gd-HP-DO3A, a gadolinium chelate complex, also known as Prohance™) and mangafodipir trisodium (a manganese chelate complex, also known as Teslascan™) are usually paramagnetic. Both gadolinium and manganese atoms contain unpaired electrons, which account for paramagnetic behavior and resultant MRI contrast.
Three-dimensional CT and MR imaging are routinely used for diagnosis of brain tumors, although these techniques do not allow the direct and gross visualization of tumor tissue with a bare eye. Further, these imaging techniques do not provide surgical guidance to surgeons for the tumor resection in real-time. Most of the unsuccessful brain tumor surgeries, which sometimes result in major neurological disabilities, were primarily responsible for the lack of technological advancement in demarcating tumor boundaries. It would thus be highly desirable to obtain a contrast agent that would meet all criteria of existing contrast agents and in addition to that it would provide real time guidance in a way that surgeons could clearly visualize the tumor during the surgical procedure.
A multifunctional contrast agent with optical, radio-opaque and magnetic properties could help in the preoperative diagnosis and the intraoperative surgical resection of brain tumors or other surgical lesions. The synthesis of bifunctional contrast agents for dual (fluorescence and magnetic) imaging was reported by others (Kircher et al, Cancer Research, 2003, 63:8122; Huber et al., Bioconjugate Chemistry, 1998, 9:242), in which organic fluorescent dyes were used. The use of organic dyes may not be suitable for real-time imaging as these dyes often undergo rapid photobleaching process.
The present invention relates to the use of luminescent semiconductor nanocrystals (quantum dots) as contrast agents and labels in biological systems. Quantum dots are nanometer scale particles that absorb light, then quickly re-emit the light but in a different wavelength and, thus, color. The dots have optical properties that can be readily customized by changing the size or composition of the dots. Quantum dots are available in multiple colors and brightness, offered by either fluorescent dyes or semiconductor LEDs (light emitting diodes). In addition, quantum dot particles have many unique optical properties such as the ability to tune the absorption and emission wavelength by changing the size of the dot. Thus, different-sized quantum dots emit light of different wavelengths. Quantum dots have been described in U.S. Pat. No. 6,207,392, and are commercially available from Quantum Dot Corporation.
Quantum dots are composed of a core and a shell. The core is typically composed of cadmium selenide (CdSe), cadmium telluride (CdTe), or indium arsenide (InAs). CdSe provides emission on the visible range, CdTe in the red near infrared, and InAs in the near infrared (NIR). The composition and the size of the spherical core determine the optical properties of the quantum dot. For instance, a 3 nm CdSe quantum dot produces a 520 nm emission, a 5.5 nm CdSe quantum dot produces a 630 nm emission, and intermediate sizes result in intermediate colors. The emission width is controlled by the size distribution.
The outer shell of a quantum dot protects the core, amplifies the optical properties, and insulates the core from environmental effects. It can also provide a surface coating to link the particles to molecules, such as polymers. Biomolecules such as antibodies, streptavidin, lectins, and nucleic acids can be coupled to quantum dots. Traditional light sources such as lamps, lasers, and LEDs are exemplary excitation sources for quantum dots.
Quantum dots are the subject of intensive investigation. In undoped II-VI semiconductors (e.g., CdSe, CdTe and ZnSe), the band gap is engineered by control of the crystal size that leads to tunable band-edge emission. By doping the nanocrystals with luminescent activators (Suyver et al., Phys. Chem. Chem. Phys., 2000, 2:5445; Jin et al., J. Lumin., 1995, 66-7:315; Behboudnia et al., Phys. Rev. B., 2001, 63:03516; Sun et al., J. Alloy Compd., 1998, 277:234; Schechel et al., Scripta Mater., 2001, 44:1213; Bol et al., J. Phys. Chem. Solids, 2003, 64:247), the excitation can be tuned by quantum size effects, even though the activator-related emission energy is largely unchanged. When a dopant with quantum states remote from the valence and conduction band edges is added to the semiconductor host, another radiative mechanism is involved. This mechanism results in localized (atomic transition) luminescence, not band edge recombination, since the luminescence emission processes are confined to the localized luminescence center.
Doped nanocrystalline II-VI semiconductors incorporating rare earth (RE) ions such as Tb3+, Eu3+, and Er3+ have been reported (Kane et al., Chem. Mater., 1999, 11:90; Ihara et al., J. Electrochem. Soc., 2000, 147:2355; Sun et al., J. Alloy Compd., 1998, 277:234; Schmidt et al., Chem. Mater., 1998, 10:65). However, due to the dissimilar chemical properties (e.g., ionic radius, valence state) between the RE ion and host cation (Cd2+, Zn2+), efficient doping of RE ions into II-VI semiconductor host is not favorable (Bol et al., Chem. Mater., 2002, 14:1121). Even though some 4fn-4fn transition-related emissions have been observed in RE ion-doped nanocrystalline II-VI semiconductors, it was speculated that their characteristic emissions originate from RE ions adsorbed on the particle surface. In contrast to RE ions, the chemical properties of Mn2+ are very similar to those of Cd2+ (or Zn2+), thus incorporating Mn2+ into II-VI semiconductor host much easier.
The Mn2+ ion, used in many luminescent materials, has a d5 configuration. The Mn2+ ion exhibits a broad emission peak, whose position depends strongly on the host lattice to changes in crystal field strength with host. The emission color can vary from green to deep red, corresponding to a 4T1-6A1 transition (Blasse et al., Luminescent Materials, Springer-Verlag, Berlin, 1994). Since this transition is not spin-allowed, the typical luminescent relaxation time of this emission is of the order of milliseconds (Blasse et al., 1994; Bol et al., J. Lumin., 2000, 87-9:315; Smith et al., Phys. Rev. B, 2000, 62:2021). Bulk ZnS:Mn has been widely used as a phosphor (Gallagher et al., J. Cryst. Growth, 1994, 138:970), particularly in alternating current thin film electroluminescence (ACTFEL) devices (Lewis et al., J. Appl. Phys., 2002, 92:6646; Wager et al., J. Lumin., 2002, 97:68; Gupta et al., Thin Solid Films, 1997, 299:33). Mn2+ d-electron states act as efficient luminescent centers while interacting strongly with s-p electronic states of the ZnS host into which external electronic excitation is normally directed. The subsequent transfer of electron and hole pairs into the electronic level of the Mn2+ ion leads to the characteristic yellow emission from the Mn2+ 4T1-6A1 transition (Gallagher et al., 1994). Possible mechanisms for excitation of the Mn2+ in semiconductor hosts (ZnS, CdS) have been suggested. In one mechanism a hole trapped by the Mn2+ ion is recombined with an electron, leading to Mn2+ in an excited state (Hoshina et al., Jpn. J. Appl. Phys., 1980, 19:279; Jaszczynkopec et al., J. Lumin., 1983, 28:319). Another suggested mechanism is recombination of a bound exciton at the Mn2+ site, which again promotes the Mn2+ to an excited state (Suyver et al., Nano. Lett., 2001, 1:429).
Although the optical properties of doped semiconductor (Mn-doped ZnS) nanocrystallines were published in 1983 (Becker et al., J. Phys. Chem., 1983, 87:4888), it was not until the publication by Bhargava's group that a considerable effort on doped semiconductor nanocrystals was made. Since Bhargava et al. reported in 1994 that Mn-doped ZnS nanocrystals exhibited dramatic lifetime shortening as well as high quantum yield (Bhargava et al., Phys. Rev. Lett., 1994, 72:416), doped semiconductor nanocrystals have been regarded as a new class of luminescent materials with a wide range of applications, e.g., in displays, sensors, and lasers (Bhargava, J. Lumin., 1996, 70:85).
Since a large portion of the atoms in nanocrystals is located on or near the surface, the surface properties should have significant effects on their structural and optical properties (Alivisatos, J. Phys. Chem., 1996, 100:13226). Organically passivated nanocrystals still have a relatively large number of unpassivated surface sites due to the limited interaction of organic passivating species with either anionic or cationic sites, resulting in partial coverage of the nanocrystal surface (Wang et al., J. Phys. Chem., 1991, 95:525). These unpassivated surface sites act as nonradiative recombination and photodegradable sites, and thus suppress the efficient luminescence and allow photodegradation of the material and devices (Peng et al., J. Am. Chem. Soc., 1997, 119:7019; Hines et al., J. Phys. Chem., 1996, 100:468). Control of the surface has been a critical issue to obtain highly luminescent, photostable nanocrystals. Inorganically passivated (or core/shell structured) nanocrystals such as CdSe/CdS, CdSe/ZnS, and ZnSe/ZnS have been reported to be an improvement over those passivated by organic surface layers (Peng et al., J. Am. Chem. Soc., 1997, 119:7019; Hines et al., J. Phys. Chem., 1996, 100:468; Lee et al., Curr. Appl. Phys., 2001, 1:169). Using inorganic materials with a wider band gap, surface-related defect states could be effectively passivated, leading to enhanced photostability as well as improved quantum efficiency.
Recently, quantum dots have been actively studied as electroluminescent (EL) components and luminescent biomarkers. Quantum dots have a tremendous potential in labeling biological entities such as cells, tissues and biohazard particles (bacteria, viruses), as evident from reports from the fields of molecular/cell biology, medical diagnostics and targeted therapeutics (Chan et al., Curr. Opin. Biotechnol, 2002, 13:40; Chan et al., Science, 1998, 281:2016; Bruchez et al., Science, 1998, 281:2013; Wu et al., Nat. Biotechnol., 2003, 21:41; Larson et al., Science, 2003, 300:1434; Dubertret et al., Science, 2002, 298:1759). The hot solution phase chemistry-derived core/shell structured Qdots, where surface passivating layers such as CdS and ZnS are epitaxially grown on the CdSe core, have been found to dramatically enhance the quantum yields of CdSe Qdots from <10% up to 40-50% (Peng et al., J. Am. Chem. Soc., 1997, 119:7019; Hines et al., J. Phys. Chem., 1996, 100:468; Dabbousi et al., J. Phys. Chem. B, 1997, 101:9463). Advanced properties such as higher quantum efficiency and improved photostability in the luminescent semiconductor Qdots open a promising possibility for the development of a new class of luminescent biomarkers. In addition, luminescent Qdots (inorganic fluorophores) have advantages over conventional organic fluorophores, since Qdots have large absorption bands, narrow spectral emission bands, and are photochemically stable. However, luminescent Qdots synthesized by hot solution phase chemistry have a poor solubility in water, resulting in a poor compatibility with biological environments and aqueous assay conditions (Chan et al., Science, 1998, 281:2016; Bruchez et al., Science, 1998, 281:2013; Wu et al., Nat. Biotechnol., 2003, 21:41; Larson et al., Science, 2003, 300:1434; Dubertret et al., Science, 2002, 298:1759; Gerion et al., J. Phys. Chem. B, 2001, 105:8861).
Due to their hydrophobic surface property, an appropriate surface coating is necessary to disperse Qdots in aqueous solution. Coating also protects them from photo-initiated surface degradation, which is directly related to fading of fluorescence intensity and toxicity. Despite recently reported toxic effects of quantum dots (Derfus et al., Nano Lett., 2004, 4:11-18), both in vitro and in vivo studies have been reported in favor of using Qdots for biolabeling applications, including in vivo disease diagnosis (Bruchez et al., Science, 1998, 281:2013-2016; Chan et al., Curr. Opin. Biotechnol., 2002, 13:40-46; Nie et al., Cytometry, 2002, 25:25; Gao et al., Nat. Biotechnol., 2004, 22:969-976).
In the visible range, because of the limitation of low signal penetration capability of Qdot fluorescence through living tissue, other types of optical probes, e.g., NIR (near infra-red) dyes, NIR Qdots, and up-converting phosphors have attracted attention recently (Josephson et al., Bioconjugate Chem., 2002, 13:554-560; Schaller et al., J. Phys. Chem. B., 2003, 107:13765-13768; Gaponik et al., Nano Lett., 2003, 3:369-372; van de Rijke et al., Nat. Biotechnol., 2001, 19:273-276; Zijlamns et al., Anal. Biochem., 1999, 267:30-36). It is expected that by using these probes, a signal from a few millimeter deep tissue (such as skin cancers) could be detected non-invasively. These NIR probes, however, will not be suitable for the detection of brain tumors. It is unlikely that an optical signal will pass through the skull, severely limiting any brain related application of these optical probes.
For in vivo bio-labeling applications, it is desired to incorporate additional properties such as radio-opacity and paramagnetism in the same probe (multifunctional probe). This will allow non-invasive tumor diagnosis using CT (Computer Tomography) scan and/or MRI (Magnetic Resonance Imaging) scan before performing the surgery. Multifunctional probes with both fluorescence and paramagnetic properties have been reported recently in the literature (Kircher et al., Cancer Res., 2003, 63:8122-8125). These probes were synthesized by incorporating fluorescent and magnetic components separately into a biodegradable polymer matrix. In these cases, organic dyes were used as fluorescent component, which may not be stable in an in vivo environment (Santra et al., Chem. Commun., 2004, 2810-2811).
There is a need for nanoparticle systems and methods that can be used to further aid in the early detection of disease. There is also a need for systems and methods that allow for high-resolution localization of biochemical activity in a living organism. There is also a need for bifunctional or multifunctional contrast agents that can be utilized in two or more different modalities concurrently or consecutively. There is yet a further need for multifunctional contrast agents that can be utilized in both MRI and optical imaging concurrently.